1. Field of the Invention
Aspects of the present invention relate to a holographic storage medium and a method and apparatus to record and/or reproduce data to and/or from the holographic storage medium.
2. Description of the Related Art
In optical holography, data is not stored on a surface of a recording medium, but is instead stored in a volume of the recording medium. A signal beam interferes with a reference beam within the recording medium, thus generating a plurality of interference gratings referred to as a data page. The interference gratings change the optical characteristics of the reference beam and cause overlapping to occur between the signal beam and the reference beam in a process referred to as “multiplexing.” When data is reproduced from the holographic storage medium, a single reference beam is incident on the recording medium under the same conditions as those conditions used during the data recording, thereby generating a diffraction beam to reproduce the stored data page. The diffraction beam is detected by a detection array, which extracts a stored plurality of data bits from a measured intensity pattern. The data page contains the data bits or pixels. As such, when the data pages overlap in the volume of the recording medium, data storage capacity can be increased.
A hologram is recorded using a signal beam S to carry data and a reference beam R to interfere with the signal beam S. As illustrated in FIG. 1A, during recording of the hologram, the reference beam R and the signal beam S interfere with each other to generate an interference pattern, which is transferred to a holographic storage medium 1. As illustrated in FIG. 1B, during reproduction of the hologram, the original reference beam R is radiated onto the recorded hologram and the recorded hologram diffracts the reference beam R, whereby the output signal beam S is generated.
Recording in the holographic storage medium 1 is carried out due to interference between a signal beam S and a reference beam R. The signal beam S is produced by a spatial light modulator which generates a page comprised of a plurality of pixels. The produced signal beam S is transmitted into an optical system and interferes with the reference beam R on the holographic storage medium 1. An interference pattern produced by the interference between the signal beam S and the reference beam R is recorded in the holographic storage medium 1. Data reproduction is carried out by radiating the reference beam R onto the recorded interference pattern to cause diffraction of the recorded signal beam S, thereby outputting the information recorded in the recorded signal beam S.
A reference beam R used for a holographic storage medium can be controlled using various configurations of components. For example, a pair of galvano mirrors can be used with other components, such as a pair of galvano mirrors and a scanner, or a pair of galvano mirrors and a translator. In such cases, a position of the reference beam R on the holographic storage medium 1 is fixed while an angle of the reference beam R changes according to a recording page. Conventionally, a plurality of pages is recorded in the holographic storage medium 1 using different angles of the reference beam R to record each of the plurality of pages, and the plurality of recorded pages forms a “book.” During reproducing, the angle of the reference beam R is also varied in order to reproduce the plurality of pages recorded on the holographic storage medium 1. Conventionally, a first page of every book is always reproduced using the same initial angle.
FIG. 2 illustrates a conventional method of changing the angle of a reference beam while reproducing pages. Referring to FIG. 2, book0 210, book1 220, book2 230, book3 240, and book4 250, each of which includes page0, page1, page2, and page3, are recorded in the holographic storage medium 1. Thus, an order of the pages recorded in each of the book0 210, book1 220, book2 230, book3 240, and book4 250 is page0, page1, page2, and page3. Hence, data must be reproduced in the same order as the data is recorded in order to properly reproduce the data from the holographic storage medium 1. In detail, page0 in book0 210 is reproduced with reference beam0, which is transmitted to the holographic storage medium 1 at an initial angle. Next, the angle of the reference beam0 is changed by a predetermined angle to generate reference beam1, and page1 in book0 210 is then reproduced using reference beam1. Next, the angle of reference beam1 is changed by a predetermined angle to generate reference beam2 and page2 in book0 210 is reproduced using reference beam2. Next, the angle of reference beam2 is changed by a predetermined angle to generate reference beam3 and page3 in book0 210 is reproduced using reference beam3. Next, the angle of reference beam3 is changed back into the initial angle to generate reference beam0, and thus page0 in book1 220 is reproduced using reference beam0.
As described above, in order to reproduce a first page in a subsequent book after all the pages in a previous book have been reproduced according to the conventional method, the angle of a reference beam must be returned to an initial position. The process of returning the angle of a reference beam to the initial position results in a delay, and decreases a data transmission rate.